I. Field
The present invention relates generally to wireless communication networks, and more specifically to spread Orthogonal Frequency Division Multiplexing (OFDM).
II. Background
The background description includes information that may be useful in understanding the present inventive subject matter. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed inventive subject matter, or that any publication, specifically or implicitly referenced, is prior art.
Communication over the spatial modes of a point-to-point MIMO channel was developed in the early 90s [e.g., S. J. Shattil, U.S. Pat. No. 6,211,671] then gradually extended to multi-user (MU-) MIMO channels [e.g., S. J. Shattil, U.S. Pat. No. 6,008,760].
In a conventional cellular communication system, transmissions to different users are formed independently. Hence, the transmission to one user can act as interference to other users. Because the system forms the transmission to each user independently, the system has no way of knowing how a transmission to a particular user will impact other users in the vicinity. As a result, interference between cells is a main factor limiting the performance of current cellular systems. For users near a cell boundary, inter-cell interference is especially problematic.
Fading and interference are the two key challenges faced by designers of mobile communication systems. While fading puts limits on the coverage and reliability of any point-to-point wireless connection, e.g., between a base station and a mobile terminal, interference in prior-art networks restricts the reusability of the spectral resource (time, frequency slots, codes, etc.) in space, thus limiting the overall spectral efficiency expressed in bits/sec/Hz/base station.
The conventional approach to interference mitigation is spatial reuse partitioning, which prevents any spectral resource from being re-used within a certain cluster of cells. Typically, the frequency re-use factor is much less than unity such that the level of co-channel interference is low. Thus, interference is controlled by fixing the frequency reuse pattern and the maximum power spectral density levels of each base station. Some CDMA systems allow for full frequency re-use in each cell, but at the expense of severe interference at the cell edge, resulting in a significant data rate drop at the terminals and a strong lack of fairness across cell users. Some interference mitigation can be achieved via limited inter-cell coordination, such as soft handover techniques. Inter-cell interference is typically treated as noise at the receiver side and is handled by resorting to improved point-to-point communications between the base station and the mobile station using efficient coding and/or single-link multiple-antenna techniques.
Some of the proposals for increasing the capacity of cellular networks include using more spectrum, increasing the number of transmit/receive antennas on each station, using dedicated beams to serve users, and micro-cell deployment. However, none of these approaches adequately address inter-cell interference, which is a primary bottleneck for spectral efficiency.
In the traditional cellular architecture, each base station only connects to a fixed number of sector antennas that cover a small area and only provide transmission/reception in its coverage area. Ideally, in such networks, the coverage areas of different base stations do not substantially overlap, as the system capacity is limited by interference. In these networks, interference makes it difficult to improve spectrum efficiency and network capacity. Another drawback to traditional cellular systems is that the base stations are built on proprietary platforms as a vertical solution.
Operators of prior-art cellular systems are faced with many challenges. For example, the high complexity of traditional base stations requires costly initial investment, site support, site rental, and management support. Building more base station sites imposes substantial infrastructure and operational expenses on the network operator. Furthermore, since the base stations can't share processing power with each other, network energy efficiency, processing efficiency, and infrastructure utilization efficiency are low because the average network load is typically far lower than the peak load. Specifically, each base station's processing capability can only benefit the active users in its cell. Thus, a base station's processing capability is wasted when the network load within its cell is low, while at other times it may be oversubscribed. Also, an idle or lightly loaded base station consumes almost as much power as it does under peak loads.
These and other drawbacks of the prior art can be reduced or eliminated by exemplary aspects of the disclosure.
As explained above, prior-art broadband wireless technologies are band-limited or interference-limited, meaning that their spectral efficiency reaches an upper limit set by the laws of Physics, such as indicated by the Shannon formulas. While the spatial domain adds another dimension to exploit via cell planning and sectoring, increasing the cell density (e.g., micro-cells, pico-cells, femto-cells) beyond a certain point fails to mitigate the performance decline as more users demand services. This is because smaller cells result in increased inter-cell interference, which establishes a practical upper bound for cell density. While the spectral efficiency of prior-art technologies is limited by the laws of Physics, the demand for data bandwidth keeps growing. As a result, today's cellular networks are already experiencing declining data rates per user.
Thus, there is a compelling need in the broadband wireless industry for systems and methods of wireless communications in which network performance is not hard-limited by the laws of Physics, but rather increases according to advances in computer processing technologies, such as indicated by Moore's law. Only systems and methods such as disclosed herein can meet the growing demands for broadband wireless data services.